TITLE: GCN CIRCULAR NUMBER: 37069 SUBJECT: EP240806a: EP-FXT detection of the X-ray emission DATE: 24/08/06 13:02:16 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn

Y. Wang (PMO, CAS), Y. J. Zhang (THU), Q. Y. Wu, Y. Liu, H. W. Pan, Z. X. Ling, C. C. Jin, C. Zhang, H. Q. Cheng, W. Chen, C. Z. Cui, D. W. Fan, H. B. Hu, M. H. Huang, D. Y. Li, H. Y. Liu, M. J. Liu, Z. Z. Lv, T. Y. Lian, X. Mao, X. Pan, H. Sun, W. X. Wang, Y. L. Wang, X. P. Xu, Y. F. Xu, H. N. Yang, W. Yuan, M. Zhang, W. D. Zhang, W. J. Zhang, Z. Zhang, D. H. Zhao (NAOC, CAS), Y. Chen, S. M. Jia, W. W. Cui, D. W. Han, C. K. Li, L. M. Song, X. F. Zhao, J. Zhang, S. N. Zhang (IHEP, CAS), E. Kuulkers, A. Santovincenzo (ESA), P. O'Brien (Univ. of Leicester), K. Nandra, A. Rau (MPE), B. Cordier (CEA) on behalf of the Einstein Probe team

Following the detection of the fast X-ray transient EP240806a (Wu et al., GCN 37063), we performed an observation of EP240806a with the Follow-up X-ray Telescope (FXT) on board the Einstein Probe (EP) mission. The observation began at 2024-08-06T05:39:11, about 1 hour after the EP-WXT detection. The exposure time is 6058 seconds. Within the error circle of the WXT source, a slowly fading X-ray source was detected at R.A. = 11.4864 deg, DEC = 5.0941 deg, with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic), which is consistent with the position of the optical source detected by GSP (Li et al., GCN 37064). The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a photon index of 2.2(-0.7, +0.7) (with a freely fitted column density value of 1.3(-0.7, +0.7) x 10^21 cm^-2). The derived average unabsorbed 0.5-10 keV flux is 1.2(-0.1, +0.1) x 10^(-12) erg/s/cm^2. There is no cataloged X-ray source within the error circle, suggesting this source being associated with EP240806a.

Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).

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